Accurate Lateral Scatter Correction Within the MatriXX

Using an IMRT MatriXX, which is used to measure intensity maps in 2D, is a promising method for the dosimetric verification of external beam, megavoltage radiation therapy. From MatriXX images, two dimensional dose distributions inside a phantom or patient can be obtained. Whereas, the lateral X-ray scatter within the MatriXX makes the accuracy in dose reconstruction dissatisfactory. Therefore, a more accurate correction for the lateral scatter contribution has to be considered. In this study, a scatter kernel was introduced to correct the dose values determined with the MatriXX. The model requires the primary dose component at the position of the MatriXX. A parametrized description of the lateral scatter within the MatriXX was obtained from measurements with an ionization chamber in a miniphantom. This yielded a good description of the lateral scatter within the MatriXX on the central axis. This is relevant since the treatment outcome critically depends on the dose delivered to the tumor (usually close to the isocenter). There is an excellent agreement between the dose values measured with the ionization chamber in a miniphantom and those obtained from the MatriXX after the scatter correction with the kernel. It can be concluded that the scatter correction is able to improve the accuracy of the MatriXX dose values on the central axis.